1. Field of the Invention
This invention relates to a process for the preparation of pure anhydrous hydrogen fluoride from anhydrous hydrogen fluoride. More specifically, this invention relates to a process for preparing pure anhydrous hydrogen fluoride by treating anhydrous hydrogen fluoride sequentially with an oxidizing agent and a reducing agent prior to distillation.
2. Prior Art
Anhydrous hydrogen fluoride is manufactured by heating a mixture of fluorspar and sulfuric acid. The main impurities in the thus prepared hydrogen fluoride are fluosilicic acid, silicon tetrafluoride, sulfur dioxide, sulfuric acid and water. These impurities are usually removed by fractional distillation and the resulting hydrogen fluoride has a purity of about 99.8% or better. However, hydrogen fluoride thus produced usually also contains minor amounts of certain other impurities which include arsenic, sulfite and iron. In the electronics industry, aqueous solutions of hydrogen fluoride are used as cleaning agents and etchants in the production of semiconductors, diodes and transitors. A high degree of purity and extremely low levels of the above impurities are required to prevent minute quantities of said impurities from remaining on the surface of said electronic industry products after they have been etched or cleaned with the hydrogen fluoride, thereby adversely affecting their function.
Several processes have been proposed for the preparation of high purity hydrogen fluoride which substantially reduce the presence of the undesirable impurities referred to above. Among such are processes that involve repeated fractional distillation in fluoroplastic equipment. However, such methods are suitable only for the preparation of small quantities of high purity hydrogen fluoride and not considered to be economical for producing commercial quantities. More attractive processes from the standpoint of commercial utility are those which are based on the conversion of the impurities to nonvolatile products by oxidizing said impurities with permanganate or chromate salts and recovering pure hydrofluoric acid with decreased impurities by simple distillation. These oxidation processes are suitable for the purification of aqueous hydrofluoric acid but they suffer from the drawback that when anhydrous hydrogen fluoride is distilled in the presence of permanganate or chromate salts, volatile manganese or chromium compounds contaminate the distillate. Thus, although the arsenic and sulfite levels are greatly decreased, the presence of large amounts of manganese or chromium, 100 ppm or higher, renders the hydrogen fluoride unsuitable for electronic and other applications which require a high degree of HF purity. A solution to this problem is claimed in U.S. Pat. No. 3,639,370 which describes a process involving the addition of an inorganic ferrous salt to the anhydrous hydrogen fluoride after the permanganate or chromate treatment to reduce excess oxidizing agents. We have found that this process results in a high level of iron contamination and consequently there is a need for a process that effectively decreases impurities in anhydrous hydrogen fluoride without introducing new impurities from the reagents used in the process.